Particularly for traffic control technology, a system is provided in which data transmission is done from a vehicle moving past a stationary beacon to that beacon. Since it is impractical to equip the vehicles themselves with a transmitter, provision is made for having carrier signals transmitted continuously from the beacon; these signals are then received by the vehicle moving past and are modulated with a data signal. The carrier frequencies in question are in the microwave range (such as 5.8 GigaHertz). The carrier signal is received by the antenna of the vehicle moving past and is modulated with a data signal furnished in the vehicle.
It is known for the modulation to be done at the base of the antenna at different frequencies for the two logical states of the data signal, and for the carrier signal to be reflected in modulated form from the antenna, so that the vehicle need not furnish transmission power of its own. The modulation methods used are typically frequency-encoding, by which the carrier frequency is shifted by a different frequency as a function of logical "1" than as a function of logical "0" (this is known as frequency shift keying).
While the stationary beacons can be set up as base installations at the necessary expense, there must be an effort to make the on-board units of the vehicles as inexpensive as possible. This is also true for the antennas that are used in combination with the on-board unit. A hindrance to this is the fact that data transmission in the system described in subject to considerable interference, which is due to other vehicles, interference from the vehicle engine and ignition system, and so forth.